1. Field of the Invention
The present invention concerns a method to generate an image with a magnetic resonance tomography apparatus and an evaluation device, as well as a magnetic resonance tomography apparatus.
2. Description of the Prior Art
Magnetic resonance tomography (abbreviated as MRT) has for many years represented one of the central imaging methods of biophysics and medicine.
MRT is based on the alignment of nuclear spins (in particular that of H atoms) in a body by means of a strong, stationary magnetic field (also called a B0 field or basic magnetic field). The nuclear spins can be deflected out of their alignment or can be affected in their alignment by energy supplied to them at a specific resonance frequency (known as the Larmor frequency) dependent on an external magnetic field (such as the B0 field).
Excited spins release energy again and generate a measurable field or response signal. The response signal emitted by the nuclear spins is detected and used for image generation.
Individual aspects of MRT according to the present invention are first discussed in detail.
a) Excitation
A number of nuclear spins are excited simultaneously. The region of a body in which nuclear spins are excited or can be excited simultaneously is designated as an “excitation region”. An excitation region is thus a region with essentially the same Larmor frequency. In order to establish or demarcate the excitation region or a sub-region to be evaluated—for example to a slice of a body to be examined—or in order to enable an association of the response signal to a spatial region (such as what is known as a voxel), a spatially-dependent coding takes place by spatially dependent shaping of the magnetic field (and therefore of the Larmor frequency), by the application or superimposition of gradient fields.
The excitation of nuclear spins takes place by supplying energy, in particular by an electrical signal or radio-frequency signal, a magnetic and/or (radio-frequency) alternating field, B1 field, external magnetic field and/or a corresponding pulse (called an “excitation signal” in the following). The excitation signal advantageously has components at the Larmor frequency.
In practice a pulse-like excitation of the nuclear spins preferably takes place, in particular with an excitation pulse. In the following an excitation signal that has a pulse-shaped curve, in particular at least one region with rising and/or falling intensity or amplitude, is designated in the following as an “excitation pulse”.
The excitation signal is advantageously generated by superimposing multiple transmission signals of different transmission devices or antennas that can form transmission channels. Particularly at higher frequencies, shadows, interferences and/or inhomogeneities occur, particularly with regard to the spatial field strength distribution. This spatial distribution of the excitation signal is designated according to the invention as a “transmission characteristic”. The inhomogeneity of the transmission characteristic of an excitation signal is relevant in excitation region. The transmission characteristic also depends on properties of the surrounding medium, such as the body to be examined or the like.
b) Acquisition
Components or portions of a response signal are also designated as an “acquisition signal”. A response signal can thus have multiple acquisition signals or be composed of these.
A response signal is advantageously acquired by means of an acquisition device. Different acquisition signals are particularly preferably acquired or, respectively, measured via multiple acquisition devices (such as acquisition coils or the acquisition device) that can also form acquisition channels.
c) Measurement Sequence and Imaging
In order to also achieve a resolution in the coding or spatial directions, a measurement process—namely excitation with specific coding and acquisition of a response signal—is implemented multiple times repeatedly with different codings. Such a sequence of multiple measurement processes is designated as a “measurement sequence”. In particular, a measurement sequence is a series of multiple measurement processes to generate an image with resolution in a phase coding direction.
In the present invention the response signals or their acquisition signals of the measurement processes or measurement sequences are advantageously stored or saved in at least one measurement matrix (also henceforth called a matrix), in particular line by line.
In each measurement process a response signal associated with this is advantageously determined. From the respective response signal a corresponding matrix line of a measurement matrix can be generated with regard to a measurement process, wherein the response signal associated with a measurement process is advantageously split up into spectral portions. The spectral portions can be respectively associated with a matrix element such that a line of the measurement matrix arises with regard to a matrix element or a response signal. In this way a measurement matrix can be successively filled with a measurement sequence.
To generate the image, the measurement matrix that is generated in such a manner is advantageously converted into an image using a Fourier transformation or another evaluation or analysis method. An image of the excitation region (such as a slice of the body to be examined) can be generated from a measurement matrix. This can have a brightness distribution that enables conclusions about the spatially-related properties of the slice, in particular with regard to the spatially-related number of H atoms, information about the chemical composition or the like. Multiple images or slice exposures of different excitation regions are possible with multiple measurement sequences.
For a 3D MRT method, analogous to the described procedure not just one very thin slice but a somewhat thicker slice or an excitation region with a certain extent in the z-direction can be excited transversal to the slice plane, in particular via an excitation signal or an excitation pulse with a corresponding bandwidth. A three-dimensional image can be generated via corresponding coding and evaluation, advantageously again with matrices (in particular multiple and/or three-dimensional matrices).
Measurement matrices and their matrix cells merely represent preferred means of representation for illustration and/or in particular mathematical and/or computational processing. The following explanations using matrices are therefore merely examples. The use of matrices does not represent a requirement for the method according to the invention, although evaluations, analyses and/or image generation are preferably implemented on the basis of matrices. Matrices enable the method necessary to generate an image to be applied in a structured and comprehensible manner. However, the present invention can also be realized with other known and future methods or representations to generate an image from response signals.
d) Image Generation at High Magnetic Field Strengths
In the presently available MRT methods the homogeneity or uniformity of the excitation is essential to the ability to interpret the image. A non-uniform excitation of the excitation region or more or less excited or, respectively, shadowed sub-regions in the excitation region lead to a non-uniform brightness distribution or even to incorrect information in the resulting image.
In order to achieve a high resolution and/or good contrasts, the use of an optimally strong magnetic field is advantageous. The Larmor frequency also simultaneously increases with an increase of the magnetic field strength. Therefore an excitation signal of correspondingly higher frequency must be used to excite the nuclear spins.
Primary magnetic resonance tomographs with magnetic field strengths between 0.5 T and 3 T are currently used. At high field strengths in the sense of the present invention, thus field strengths that are advantageously above 3 T (for example 7T), the Larmor frequency is so high that it increasingly leads to diffraction effects and/or shadows in the body to be examined. In particular, this leads to diffraction effects and distortions of the transmission characteristic. This results in a non-uniform and/or insufficient excitation of the nuclear spins in the excitation region. Therefore different methods (in part very complicated methods) have been developed in order to achieve an optimally homogeneous excitation of a slice or, respectively, of the excitation region, in particular even given use of higher magnetic field strengths.
Methods to homogenize fields to excite nuclear spins today operate most often with transmission devices with multiple transmission units (such as antennas or coils) that can be operated independent of one another, thus with multiple (independent) channels. Channels can be associated with both transmission and acquisition devices. Therefore differentiation is also made in the following between transmission channels that advantageously correspond to transmission devices and acquisition channels that advantageously correspond to acquisition devices. Since it is possible to also use transmission devices as acquisition devices and vice versa, it is possible that multiple channels—in particular one transmission channel and one acquisition channel—can be associated with a transmission and/or acquisition device. Methods with multiple transmission devices or, respectively, multiple transmission channels that can be operated independent of one another are also called multichannel transmission methods. Methods with acquisition devices with multiple acquisition units are henceforth accordingly called multichannel acquisition devices.
In the case of multichannel transmission methods, the excitation signal advantageously consists of multiple transmission signals that are in particular associated with transmission devices or channels. Given multichannel acquisition methods, the response signal can have multiple acquisition signals that in particular are associated with acquisition devices or channels.
In a multichannel transmission method known from DE 101 24 465 A1 (corresponding to U.S. Pat. No. 6,900,636) from which the present invention arises, amplitude and phase positions of different transmission signals or, respectively, for different transmission devices are preset in a fixed manner such that an optimally homogeneous transmission characteristic—thus an optimally homogeneous B1 field—is achieved. The corresponding adaptation of the transmission signals is also called “radio-frequency shimming” or “RF shimming”. However, given the use of high magnetic field strengths the method is in particular not in the position to sufficiently reduce shadows.
In another multichannel transmission method (for example that known from DE 10 2004 002 009 A1, corresponding to U.S. Pat. No. 7,218,113) it is sought to affect the transmission characteristic via transmission signals with pre-distorted and temporally variable pulse shapes so that a homogeneous excitation of the excitation region is achieved. These methods are also called “Transmit Sensitivity Encoding” or “Transmit SENSE” but are significantly more complicated in comparison to RF shimming. A separate, more complex pulse generator that supports a variation of amplitude and phase in the time curve of the pulse is required for each transmission device or, respectively, each transmission signal. A similarly complicated determination of the properties of each individual transmission device in advance of each measurement is required for calibration, which leads to time losses in the measurement.
A method known from DE 10 2005 018 937 A1 (corresponding to United States Publication No. 2007/0013374) addresses the calculation of calibration coefficients for Transmit SENSE, wherein individual coils of a transmission coil array are activated individually and corresponding response signals are acquired simultaneously by means of multiple methods. The known method does not address an image generation with multiple transmission modes, nor is the large system cost to implement Transmit SENSE reduced.
WO 2009/053770 A1 (corresponding to United States Publication No. 2010/0301856) discloses a transmission method that resembles Transmit SENSE. However, in contrast to Transmit SENSE constant phases and amplitudes are used at least in certain time segments, which should reduce the complexity of the system.
DE 10 2005 039 686 B3 (corresponding to United States Publication No. 2009/0309594) describes a method to improve contrasts of MRT images. Fields are used that are advantageously right-polarized since this abets the transfer of energy to the spins. In particular at high Larmor frequencies, the acquisition of left- and right-polarized fields takes place since both polarization types can contain information due to distortion effects. However, shadows can also not be effectively prevented or compensated with this method.
An additional limiting factor in MRT is the acquisition time necessary to achieve high-resolution images. In order to optimally reduce the residence time of a body to be examined (such as that of a person) while maintaining resolution and maintaining contrasts, methods have been proposed for partial parallel acquisition (henceforth abbreviated as PPA), wherein additional information from multiple acquisition channels or, respectively, acquisition signals that are independent of one another are used. In this way it is possible to omit measurement processes and to reconstruct the image on the basis of this independent information of multiple acquisition channels in order to achieve the same resolution—in particular in the direction of the time-consuming phase coding (in what is known as k-space)—with a reduced number of measurement processes and accordingly in a shorter amount of time per measurement sequence.
WO 2008/004192 A2 (corresponding to United States Publication No. 2009/0292197) discloses a method to accelerate the acquisition of MRT data in a PPA measurement sequence, wherein a characterization of the transmission units is necessary to process the acquisition signals. The goal of the described method is to reduce the correction cost and the cost to determine corresponding correction data in that a reduced resolution is used for a reference measurement.
DE 10 2005 018 814 A1 (corresponding to U.S. Pat. No. 7,495,437) provides a determination of a special construction matrix by means of which a processing of incompletely acquired data sets or measurement matrices can take place. The computing cost to generate an image under the described requirements should be reduced in this manner.
DE 101 44 654 A1 (corresponding to U.S. Pat. No. 6,734,673) proposes a reconstruction of unacquired lines of a measurement matrix using a sensitivity function for individual acquisition directions or channels, wherein this is described by Fourier series.
From the prior art, at present only Transmit SENSE or its modifications are in the position to effectively reduce or avoid a shadow image given the use of high magnetic field strengths (such as 7 T). Conversely, complicated and expensive devices as well as complex signal processings are required to implement such methods.